This invention is related to a bubble-operated pump for recirculating liquid metal in a bath of molten metal. It is desirable to circulate the metal in a bath of molten metal, such as of aluminum, used to coat steel and the like. The high temperatures and the chemical and physical natures of the metal make it difficult to provide a long-lived mechanical pump.
Several versions of a bubble-operated pump for such purpose are illustrated in U.S. Pat. No. 5,203,910 which was issued Apr. 20, 1993 to Larry D. Areaux and Brian Klenoski for "Molten Metal Conveying Means and Method of Conveying Molten Metal from One Place to Another in a Metal-Melting Furnace." The Areaux, et al. patent shows a pump having an inclined conduit for drawing metal from the bottom of the pot and passing it upwardly to a discharge point below the metal line. The metal flow is induced by introducing nitrogen through a gas feed pipe. Areaux preferred that the pipe be made of metal clad with ceramic, or more advantageously a graphite clad with ceramic. Such pipe is difficult to make and has a relatively short life.
Pipe metal will dissolve in the molten metal, and the graphite will burn. In addition, the difference in the coefficients of expansion between the ceramic and the graphite will make both approaches fail following immersion in the bath. The inclined conduit design, in addition, is not realistic since the support and gas transmit must occur at a single point.
Tests conducted on the Areaux design shown in FIGS. 15 and 16, (the only configuration that could actually be fabricated) demonstrated that separation of the different materials generated gas leakage capable of completely eroding the pump graphite components in a matter of hours.
The sharp transition required at the top of the inclined tube created cavitation. The gas pressure and temperature in this area overcame the impact resistance of the ceramic, causing it to break.
The Areaux design must use cylindrical tubes, not an elliptically shaped tube, in order to warrant some sort of seal between the ceramic and graphite or steel components. To increase the pump flow, his design requires a plurality of cylindrical inclined tubes manifolded by a graphite, steel or refractory housing with the obvious manufacturing and operational complications.